Attention has been drawn to fuel cells using hydrogen and oxygen, as a power generating system which presents substantially no adverse effects on the global environment because in principle, its reaction product is water only. Among them, in recent years, a polymer electrolyte fuel cell using a proton conductive ion exchange membrane as an electrolyte is expected to be useful for in-vehicle power sources, since its operation temperature is low, its output density is high and it can be downsized.
A polymer electrolyte fuel cell is characterized in that its operation temperature is low (from 50 to 120° C.) as mentioned above. On the other hand, however, it has a difficulty such that exhaust heat can hardly be utilized effectively for e.g. an auxiliary power. In order to offset such a difficulty, the polymer electrolyte fuel cell is required to have a high performance in utilization of hydrogen and oxygen, i.e. high energy efficiency and high output power density.
In order that the polymer electrolyte fuel cell satisfies the above requirement, a gas diffusion electrode (usually composed of a catalyst layer containing a catalyst and a gas diffusion layer comprising e.g. a carbon paper and a carbon cloth) and a membrane/electrode assembly having such electrodes formed on both surfaces of an ion exchange membrane, are particularly important among elements constituting the cell. Heretofore, a catalyst layer of the gas diffusion electrode is prepared from a viscous mixture obtained in such a manner that a catalyst powder for facilitating electrode reaction and a fluorine-containing ion exchange resin for increasing conductivity and preventing clogging (flooding) of a porous body due to condensation of water vapor are dissolved or dispersed in an alcohol solvent such as ethanol. Then, such a viscous mixture is directly applied on the surface of an ion exchange membrane, or separately applied on a substrate sheet to obtain a layer, which is transferred or bonded on the surface of the ion exchange membrane to form a gas diffusion electrode. However, a gas diffusion electrode obtainable in such a manner is not necessarily sufficient for satisfying the gas diffusion property, the electrical conductivity, water repellency and durability, and further, in a case where the above viscous mixture is applied on the ion exchange membrane or the substrate sheet to prepare the gas diffusion electrode, there was such a problem that e.g. the coating property was not necessarily good.
Further, heretofore, a polymer electrolyte fuel cell using such a gas diffusion electrode is not also sufficient for satisfying the properties, and especially, e.g. the output current density is required to be further improved.
In order to solve the above problems, for example, a method has been known, in which a fibrous material such as carbon fiber is incorporated, as a pore-forming agent, in a catalyst layer so as to suppress porosity of electrodes and increase the electrical conductivity at the same time, (see Patent Document 1). However, there has been a problem that it is impossible to obtain a substantially high cell voltage at a high current density by the above method.
Patent Document 1: JP-A-2005-26174